1. Field of the Invention
The present invention relates generally to the field of fluid transport, and more particularly to a method and apparatus for venting entrained gas from pumped fluids.
2. Description of Related Art
Canned motor pumps are frequently used in processes in which release of the process fluid from other types of pumps, such as sealed type pumps, presents a process, safety or environmental hazard. Because canned motor pumps are fully enclosed they are preferred over sealed pumps which rely on a mechanical seal or shaft packing to prevent the release of the process fluid. FIG. 1 shows a typical canned motor pump 10 according to known form. An electric motor 12 drives a pump impeller 14 to pump a process fluid from a suction port 16 to a discharge port 18 in a pump housing 20. The motor 12 is housed within a fluid-tight motor housing 22 and a can 23. In some applications, a portion of the pumped process fluid is bled off adjacent the discharge port 18, and circulates through a cooling fluid circuit 24. The fluid circulated through the cooling fluid circuit 24 is directed through the bearings 26 and around the rotor 27 to cool and lubricate the motor 12 and bearings 26. An auxiliary impeller can be provided, typically within the motor section, to circulate the cooling fluid through the cooling fluid circuit. U.S. Pat. No. 4,616,980 to Carpenter and U.S. Pat. No. 5,397,220 to Akihisa et al. are incorporated herein by reference for general background information regarding canned motor pumps. In other applications, a segregated cooling fluid circuit is provided. Rather than bleeding off a portion of the process fluid, a separate non-process coolant fluid is circulated through a segregated cooling fluid circuit to cool and lubricate the motor and associated bearings. U.S. Pat. No. 5,256,038 to Fairman is incorporated herein by reference for additional background information. Canned motor pumps are commercially available from several manufacturers including Heyward Tyler, Sundyne and Hermetic.
Heretofore, canned motor pumps, and to some extent sealed pumps as well, have suffered certain disadvantages. For example, during use, the pumps circulate process fluids along fluid paths that, in many instances, contain expansion tanks that are commonly blanketed with inert gases. Inert gases suitable for blanketing processing fluids include any gas which is nonreactive with the cooling and/or process fluids in use. Suitable inert gases include nitrogen, argon, and the like. As the process fluids cool in the tank, inert gas is absorbed into the process fluid. Inert gases may also be introduced during shutdown and startup of a process, as various valves are opened or closed and the like. Upon heating of the gas, absorbed inert gas forms entrained bubbles, which frequently accumulate in a pump, particularly at the pump shaft because of the centrifugal forces. Accumulation of gases at the shaft can cause mechanical failures in sealed type pumps and permits gas to migrate into the cooling circuit in canned motor pumps. Gas entrainment and accumulation in canned motor pumps can cause cavitation of the auxiliary cooling impeller, causing loss of cooling fluid to the motor which can result in motor failure.
Prior attempts to address this problem have included forcing process fluid from the discharge of the impeller into the canned motor and back through the eye of the impeller. While this may slow the collection of entrained gas around the auxiliary cooling impeller, the problem is not entirely solved. Accumulation of gas, even at a relatively slow rate is generally not acceptable because the pump, and therefore the process, must be stopped periodically to vent accumulated entrained gas, thereby reducing process efficiency.